Telephone answering systems are available on the market which respond to ring signals received over the telephone line to become activated so that a recorded announcement may be transmitted over the telephone line to the calling party and so that a message from the calling party may subsequently be recorded.
However, problems have arisen in the past in making such telephone answering systems unresponsive to other signals on the telephone line, such as dialing pulses, or multi-frequency dialing tones. For example, if a person attempts to dial a telephone in the vicinity of an activated prior art telephone answering system, the answering system has a tendency to respond to the dialing pulses or tones generated during the telephone dialing operation, mistaking them for ring signals. This causes the answering system to come on to the telephone line and to break the outgoing dialing signals so that no outgoing calls can be completed.
The ring detector circuit of the present invention, accordingly, has utility in conjunction with telephone answering systems, and it will be described in such an environment. However, it will become evident as the description proceeds that the detector circuit of the invention is not limited to such a use, but finds utility in conjunction with a number of systems in which incoming signals are to be distinguished, and only one type of incoming signal is to be recognized and detected.
Distinguishing between ringing signals and dial pulse signals on the telephone line has presented a problem in the prior art telephone answering systems, because the bursts of ringing signal and the trains of dial pulses closely resemble one another when detected at a bridged point on the telephone line.
For example, the telephone ringing signal is typically a high voltage wave, which may be either square or sinusoidal in shape, and which has an amplitude of approximately 40-100 volts RMS, or approximately 113-282 volts peak-to-peak; and of a frequency which may range from 16 Hz to 67 Hz.
Dial pulse signals, on the other hand, typically have a waveform which is relatively square in shape with a maximum amplitude of approximately 100 volts peak-to-peak, and of a frequency which may range from 8 Hz to 20 Hz.
It is, therefore, impossible to distinguish between the ringing signals and the dialing signals on the basis of frequency or wave shape, since both these parameters may be the same in some instances. It is also difficult to differentiate between the ringing and dialing signals on the basis of voltage amplitude since there is usually only a 13% differential therebetween, and this differential is far too critical to permit reliable differentiation between the two signals on an amplitude basis.
The detector circuit of the present invention distinguishes between the ringing signals and the dialing signals by counting the number of pulses or cycles of each burst of ringing signal, and by counting the number of pulses in each train of dialing signal. The detector circuit produces an output only when the number of cycles in a particular burst of ringing signal indicate that the signal is actually a ringing signal and not a dialing signal.
The number of pulses in each train transmitted over the telephone line during a dialing operation is very closely controlled, because the number of pulses in a particular train is a function of the digit dialled. For example, the largest digit dialled (0) is represented by a train of 10 pulses. The time between the dialing of successive digits is typically about 600 milliseconds. Therefore, the most severe condition presented to the telephone line during the dialing operation, insofar as distinguishing the dialing signal from the ringing signal is concerned, is the transmission of a series of ten pulses during the dialing operation, followed by an interruption of 600 milliseconds, followed by a second series of 10 pulses, followed by another 600 millisecond interruption, followed by yet another train of 10 pulses, this being repeated as often as the digit (0) is dialled.
A typical ringing signal transmitted over the telephone line, on the other hand, comprises bursts of the ringing signal of 1-3 seconds duration, and interrupted by time intervals of 2-5 seconds. In the most severe case, insofar as distinguishing the ringing signal from the dailing signal is concerned, a burst of ringing signal will have a minimum of 16 cycles, representing the 16 Hz minimum frequency, and will have a 1 second minimum duration.
The system of the present invention is predicated on the concept of counting the number of pulses transmitted over the telephone line during each signalling operation to differentiate between the ring signals and the dialing signals. In the circuit of one embodiment of the invention, a counter is enabled at the beginning of each train of dialing pulses and at the beginning of each burst of ringing signal. The counter then counts the number of pulses or cycles transmitted over the telephone line during each dialing or ringing signal operation. If an interruption of greater than 400 milliseconds between successive trains or bursts of a received signal is encountered, the counter resets and begins counting again when the next train of pulses or burst of signal appears on the telephone line. Only when the counter reaches a count of 12, or greater, does the circuit of the invention produce a ring-detect output.
Thus in the presence of a dialing signal the counter counts the first train of pulses corresponding to the first digit dialed (maximum of 10), and resets during the interruption (600 milliseconds) between the first digit dialled and the second digit dialled, counts the number of pulses in the second train corresponding to the second digit dialled (maximum of 10), and so on. At no time during the reception by the detector circuit of the dailing signal does the counter reach the count of 12, which is necessary for the circuit to produce a ring detect output signal. Therefore, the circuit is unresponsive to the dialing signal.
When the ring signal is transmitted, the counter of the detector circuit of the invention responds to the first ring signal bursts and counts the cycles thereof (a minimum of 16), resets in the interval between the first and second burst, counts the cycles of the second burst (a minimum of 16, and so on. For each counting operation, the counter exceeds the count of 12, and a ring detect output signal is produced indicating that a ring signal has been detected.